1. Field of the Invention
This invention relates to a pneumatic tire having an excellent bead portion durability, and more particularly to a heavy duty pneumatic radial tire for use in truck, bus and the like.
2. Description of Related Art
In the conventional heavy duty pneumatic radial tire, it is general that as partly and sectionally shown in FIG. 1, at least one carcass ply c is toroidally extended between a pair of bead cores bc embedded in the respective bead portions b through a tread portion and sidewall portions and turned up around the bead core bc from an inside of the tire toward an outside thereof in a widthwise direction of the tire. In such a structure of the bead portion, when the tire is assembled onto a rim and run under loading, a part of the bead portion located outward from a rim flange rf in a radial direction is repeatedly subjected to a deformation falling down outward in a widthwise direction of the tire, whereby stress concentration is caused in a turnup end of the carcass ply c and hence there is caused a problem that separation failure is apt to be caused at a turnup end of the carcass ply.
In order to solve the separation failure at the turnup end of the carcass ply, it is usual to arrange a bead portion reinforcing layer along an outside of the turnup portion of the carcass ply so as to extend over the turnup end. This bead portion reinforcing layer is a rubberized steel cord layer usually called as a wire chafer. The arrangement of such a reinforcing layer is advantageous to control the bending deformation of the bead portion to prevent the separation failure at the turnup end. However, stress is apt to concentrate in an end position of the reinforcing layer and there is caused a fear of creating troubles such as cracking of the bead portion and the like in the end position, and hence it becomes impossible to effectively enhance the bead portion durability.
As the tendency of flattening the tire is recently increasing, input to the bead portion becomes more severe and hence the control of separation failure at the turnup end is insufficient even when the bead portion is reinforced with the reinforcing layer.
In order to prevent the occurrence of separation failure at the turnup end, therefore, it is proposed to wind the turnup portion of the carcass ply around the bead core to locate the turnup end thereof in a zone of the bead portion having a small strain without using the bead portion reinforcing layer as disclosed in JP-A-9-156310. In this case, the strain concentration at the wound end of the carcass ply is largely mitigated to prevent the occurrence of separation failure at the wound end. However, the turnup portion of the carcass ply is replaced with the portion of the carcass ply wound around the bead core, whereby the resistance of the bead portion to the fall-down deformation is decreased and fatigues in an outer surface part of the bead portion contacting with a vicinity of an outer peripheral edge of a rim flange and in a bead heel part contacting with a base part of the rim flange are particularly increased, and hence there is caused a problem that fatigue breakage from these parts can not be eliminated.
It is, therefore, an object of the invention to advantageously solve the aforementioned problems of the conventional techniques and to provide a pneumatic tire having an excellent bead portion durability by preventing the occurrence of separation failure at the wound end of the carcass ply and effectively eliminating the fatigue breakage of the bead portion.
According to the invention, there is the provision of in a pneumatic tire comprising a tread portion, a pair of sidewall portions and a pair of bead portions and a carcass toroidally extending between a pair of bead cores embedded in the respective bead portions and comprised of at least one rubberized carcass ply containing steel cord(s) therein, in which the carcass ply is wound around the bead core from an inside of the tire toward an outside thereof in a radial direction to form a part or a whole of a wound portion of the carcass ply as a wind contact part along the peripheral face of the bead core, the improvement wherein at least one steel cord reinforcing layer is arranged in the bead portion.
The carcass ply in the invention includes a carcass ply containing many steel cords arranged at a cord angle of about 70-90xc2x0 with respect to an equatorial plane of the tire and a carcass ply formed by folding a single steel cord at a position corresponding to the wound part of the carcass ply and extending in the circumferential direction of the tire while detouring. Moreover, in the formation of the wind contact part, it is favorable that the carcass ply is subjected to plastic deformation in at least one of positions corresponding to corner parts of the bead core.
The term xe2x80x9csteel cord reinforcing layerxe2x80x9d used herein means a bead portion reinforcing layer made of rubberized steel cords. The arranging region of the steel cord reinforcing layer may be arranged either inside and outside the bead core in the widthwise direction of the tire including an inner peripheral side of the bead core.
According to the above bead portion structure in the pneumatic tire according to the invention, the occurrence of separation failure at the wound end of the carcass ply can be prevented by winding the side portion of the carcass ply around the bead core along the outer peripheral face thereof. And also, the given number of the steel cord reinforcing layers are arranged in the required region of the bead portion to enhance the lateral rigidity of the bead portion, whereby the outward fall-down deformation of the bead portion in the widthwise direction of the tire can effectively be controlled in the running of the tire under loading or the like to sufficiently prevent the occurrence of fatigue breakage of a part of the bead portion contacting with a rim flange or the like.
In a preferable embodiment of the invention, when using two or more steel cord reinforcing layers, a main body of the carcass ply other than the wound portion is sandwiched between two steel cord reinforcing layers in a thickness direction thereof. In this case, steel cords constituting one of the steel cord reinforcing layers are extended in a direction opposite to at least one of a cord extending direction of the other steel cord reinforcing layer and a cord extending direction of the carcass ply.
When the main body of the carcass ply is sandwiched between the two steel cord reinforcing layers, the lateral rigidity of the bead portion can be more enhanced as compared with the case that one steel cord reinforcing layer is arranged along the inside or outside of the main body of the carcass ply in the widthwise direction of the tire, whereby the fear of causing the fatigue breakage of the bead portion can effectively be eliminated to largely improve the bead portion durability. This is particularly conspicuous when the cord extending directions are opposite between the steel cord reinforcing layers and/or between the steel cord reinforcing layer and the carcass ply. In the latter case, the rigidity of the bead portion can be further increased.
In another preferable embodiment of the invention, the steel cords constituting the steel cord reinforcing layer have a cord diameter of 1.00-1.50 mm, and a free end of the steel cord has a flare having a scattering width within a range of 1.0-1.5 times the cord diameter, and the steel cords are arranged in the steel cord reinforcing layer at a distance between the cords of 1.00-1.50 mm in a direction perpendicular to a longitudinal axis of the cord, and the steel cord constituting the steel cord reinforcing layer is a Z-lay outer-sheath structure.
The term xe2x80x9cscattering width of flarexe2x80x9d used herein means a component of a longitudinally scattered length perpendicular to the cord axis.
In the other preferable embodiment of the invention, a start end of the steel cord reinforcing layer located at a side of a main body of the carcass ply is arranged so that a shortest distance (L) measured outward from a normal line (n) drawn at a first rim line position to an outer face of the bead portion in the radial direction of the tire is positioned within a range of 15-25 mm, while a terminal end of the steel cord reinforcing layer located at a side of the wind contact part of the carcass ply is arranged so as to position within a range sandwiched between a vertical line (m) drawn from an outermost end position of the bead core in the radial direction to the outer face of the bead portion and the normal line (n).
The term xe2x80x9cfirst rim line positionxe2x80x9d used herein means an outermost end of an outer surface portion of the tire contacting with a rim in the radial direction when the tire is assembled onto a standard rim and a maximum air pressure and a maximum load capacity are applied thereto.
In a further preferable embodiment of the invention, at least one organic fiber chafer is arranged at the side of the wind contact part of the carcass ply so as to cover the terminal end of the steel cord reinforcing layer. In this case, the organic fiber cords constituting the organic fiber chafer are arranged at a cord angle of 15-75xc2x0 with respect to an arranging direction of the steel cords constituting the steel cord reinforcing layer. Thus, the occurrence of separation failure at the terminal end of the steel cord reinforcing layer can be more controlled.
In a still further preferable embodiment of the invention, a cushion rubber layer is interposed between the main body of the carcass ply and the start end portion of the steel cord reinforcing layer. In this case, the cushion rubber layer at the position of the start end of the steel cord reinforcing layer has a rubber gauge of 1.5-2.0 mm viewing a section in a widthwise direction of the tire. Thus, the occurrence of separation failure at the start end of the steel cord reinforcing layer can be more controlled.